This proposal is focused on the interactive effects between soybean phytochemicals and tamoxifen (TAM) on breast cancer (BRCA). The hypothesis is that soy phytochemicals, especially soy isoflavone genistein, may counteract the inhibitory effect of TAM on estrogen-dependent BRCA, but may inhibit the growth of TAM-resistant BRCA and synergize the inhibitory effect of TAM on estrogen-independent BRCA. TAM has become the first-line treatment for estrogen-dependent BRCA, while its effect on estrogen-independent BRCA is limited. The positive response is usually of short duration, and most tumors eventually develop TAM-resistance. Soy phytochemicals, especially soy isoflavones, have anti-tumor activities. On the other hand, soy isoflavones are weak phytoestrogens and may interfere with TAM in competing with the binding sites of estrogen-receptors and modulate estrogen-mediated transcription. With more and more women are taking TAM and soy phytochemical supplements, it is imperative to evaluate the potential interactions between TAM and soy phytochemicals before any guideline and recommendations can be made. Our preliminary studies have found that soy isoflavone genistein counteracts the inhibitory effect of TAM on estrogen-dependent breast cancer cells, but may synergize the inhibitory effect of TAM on estrogen-independent breast cancer cells. Specific Aim 1 is to determine the combined effect of soybean components and TAM on the growth of estrogen-dependent human breast tumors. MCF-7 human BRCA cells will be implanted into mammary fat pad of immune deficient mice to develop estrogen-dependent breast tumor model to evaluate the interactive effects between a novel soy phytochemical concentrate or soy isoflavone genistin and TAM on MCF-7 tumor growth. Specific Aim 2 is to determine the effect of soybean bioactive components on the growth of TAM-resistant human breast tumors. A clinically relevant TAM-resistant human breast tumor model will be applied to evaluate the effects of soybean components on the growth of TAM-resistant breast tumor. Specific Aim 3 is to determine the combined effect of soybean components and TAM on the growth of estrogen-independent human breast tumors. MDA-MB-435 human BRCA cells will be implanted into mammary fat pad of immune deficient mice to develop an estrogen-independent breast tumor model to evaluate the combined effects of soy components with TAM on estrogen-independent BRCA. A series of biomarkers will be determined to elucidate the possible mechanisms of action. We will first determine the biomarkers that are related to estrogen-pathways such as tumor expressions of ER(alpha), ER(beta) and serum levels of estrogens. We will then quantify tumor proliferation index by PCNA staining and apoptotic index by TUNEL assay. We will quantify tumor microvessel density (factor VIII staining) as an angiogenesis marker and the expression of angiogenesis factors, basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF). These in vivo bioassays and mechanistic studies will be expected to provide significant insight into the future consideration of potentiating the efficacy of TAM on BRCA treatment and prevention by soy bioactive component-containing supplements.